Firing the top barrel without the hammer dropping is very strange!! The only possible way I can visualize that happening with two different trigger assys is a sticking firing pin and possibly a worn pin hole? The weight of a loaded round would move back with enough force to ignite the primer where it wouldn't with just the primed case? Still would need enough head space for the pin to protrude enough for that to take place too!! Let us know what you find out please?

Paul, if the pin moved forward when the bottom barrel fired, lodged/stuck in a worn hole, possibly recoil alone could cause the second firing with a full load and not fire the primed hull! I'd look for excess head space also.The only plausible explanation I can see!

Glad to hear you've solved your dilemma guy!! Like you, I too have seen the marking on the top primer in my P guns. Looks as though the carbon build-up in the receiver holes may have been the primary cause.

Just another reminder for me to keep my receiver cleaned better also!! Time too to have Marcello send me a couple hammer springs and his new firing pins!!